1. Field of the Invention
The present invention relates to a method for manufacturing miconductor device. More particularly, it relates to a method for manufacturing a semiconductor device having a via hole for electrical connection between upper and lower electrodes.
2. Description of Prior Art
Generally, a semiconductor device has a via hole through which current flows from an external power supply into an active region of an insulating layer formed on a semiconductor device which is deposited with a first metal layer on its lower surface. FIGS. 1A to 1C depict the conventional steps in the manufacture of the semiconductor device with a via hole.
Referring to FIG. 1A, a first insulating layer 21 is deposited on a semiconductor substrate 20 having a first metal layer on its lower surface (not shown), and a second metal layer 22 such as aluminum is deposited onto the insulating layer 21 for the formation of metal lines. In order to prevent irregular reflection from the second metal layer 22 deposited onto the first insulating layer 21 when performing a photolithographic process, a third metal layer 23 is formed on the second metal layer 22. A first photoresist layer 24 is then coated on the third metal layer 23 to serve as an etching mask for the formation of the metal line. Thereafter, the second metal layer 22 and the third Metal layer 23 are etched away, using the first photoresist layer 24 as an etching mask, whereby a metal line is formed by the metal layers 22 and 23 on the insulating layer 21 as shown in FIG. 1B.
Referring now to FIG. 1B, a second insulating layer 25 is deposited over the first insulating layer 21 to surround both of the second metal layer 22 and the third metal layer 23. A second photoresist layer 26 is coated on the second insulating layer 25, and the metal layers 22 and 23 are etched using the pattern of the second photoresist layer 26 as an etching mask, so that a via hole 27 is formed in the semiconductor device as shown in FIG. 1C.
With the formation of the via hole 27, however, is the formation of an undercut 28 located at the bottom area of the via hole 27, on the upper portion of the metal layer 22. The undercut 28 is caused by excessive etching of the metal layers 22 and 23. When a metal line is formed on the second metal layer 22 over the undercut 28, the metal line becomes too thin around the edge of the bottom of the via hole 27 to achieve good step coverage, which may result in disconnection of the metal line to make matters worse. Thefore, the undercut is one of the principal causes of the low reliability problem.